When the cyclic variation of the hydrocarbon content of exhaust gases is studied in detail, it is noted that there are peaks of high hydrocarbon concentration immediately when an exhaust valve opens and just before the exhaust valve closes. In a well tuned engine, the hydrocarbon concentration between these two peaks is significantly lower and within the range expected from complete combustion. The two peaks are not therefore caused by an incorrect fueling map and other reasons must be the cause of the presence of unburnt fuel in the exhaust.
The present invention is concerned with the cause of the second peak which occurs at the end of the exhaust event rather than the first peak. It is generally believed that a major cause of this problem is the presence in the combustion chamber of small crevices into which fuel can be compressed but into which the combustion flame cannot penetrate. One such crevice is that surrounding the piston top land that is to say the small space between the piston and the cylinder above the top piston ring. During the compression stroke, fuel and air are compressed into this space. During combustion, the expanding flame front pushes mixture ahead of it into this crevice tending to increase the amount of fuel stored even further. However, the flame cannot enter this crevice because it is bound by two cold walls and the flame is quenched during its attempt to penetrate into this gap. Consequently, a quantity of fuel remains trapped in the crevice throughout the power stroke until the pressure in the combustion chamber during the exhaust stroke drops to allow the unburnt charge to escape from the crevice. The unburnt charge will then reside near the top of the piston and will be discharged towards the end of the exhaust stroke.
Attempts have been made in the prior art to reduce the crevice volume by reducing the distance between the piston crown and the top ring but this causes problems because the top ring then runs hotter and reduces engine life.